This invention relates to a method and apparatus for waste heat recovery and reduction in industrial furnaces and heating equipment.
Three alternative methods for recovery of latent heat of flue gases escaping from high temperature furnace working chamber are common: (a) use of waste heat to preheat combustion air; (b) use of waste heat for initial preheating of the work; and (c) use of hot flue gases for other low temperature operations such as drying or steam generation.
All three methods have come into wide use during the past decade. Steadily rising fuel prices have continually lowered the minimum size of furnace which can justify use of these heat recovery methods. Combustion air preheating by recuperation is the most common method in use today because of the relatively low capital and operational costs involved. Recuperation also has the advantage of a good relationship between the availability of waste heat and the need for that heat in the incoming combustion air.
Initially, the general concept of combustion air preheating was accomplished by means of a central external recuperator, hot air ducts and a set of hot air burners for heating the furnace.
More recently, the most common choice of heat recovery for industrial furnaces is utilization of an individual recuperator for each hot air burner. This has made it possible to reduce the cost of both recuperators and hot air burners through volume production. As used in this manner, the recuperator can be either radiative, radiative-convective, or convective. It can be made of metal or refractory materials.
Further development of this general concept has resulted in several specific improvements such as selfrecuperative burners and side-by-side recuperator-burners, which have the common design approach of reducing the cost of the system by combining the recuperator and burner into one integral unit, eliminating the need for hot air ducting and in some cases reducing the amount of insulating material involved. However, these approaches have the obvious limitations of restricted capacity, limited heat recovery and large physical size for a given capacity. Although many self-recuperative burners have been designed, few are used with success. The major disadvantage of these burners is that positioning the flue outlet very close to the flame envelope results in losing a significant part of the hot combustion product before its heat has been transferred to the load.
Another approach is the internal recuperator, which consists of stainless steel piping located in the existing furnace exhaust flue. Hence, this design uses the exhaust flue as the housing for a bundle of pipes which contain the combustion air being heated. The advantage of this approach is the utilization of the flue duct as the housing for the heat-exchanger, but the disadvantages are: a high pressure drop resulting in poor circulation inside the furnace chamber, limited surface area of the heat exchanger and therefore reduced heat recovery, a complicated flue channel structure, and low durability of the castable refractory dividing wall between the flue channel and the furnace chamber.
In addition, it is desirable to prevent the loss of heat through the walls of the furnace, by the use of walls with good insulation. Historically, this has been accomplished by means such as constructing the walls of the furnace with brick. The development of new ceramic fiber based insulating materials having high thermal resistance, low density and specific heat made it possible to substitute heavy refractory furnace linings for light metal structures lined with ceramic blanket or blocks. This current improvement has changed the traditional design of furnace walls and has made it possible to fabricate furnace walls with prefabricated modules containing mild steel plate lined with ceramic fiber materials.